Environmentally Benign Photoreactions for Hydrogen Production and Cleavage of N═N bond in Azobenzene over Co-Doped Zn(O,S) Nanocatalyst: The Role of In Situ Generated H+

In this study, fabrication of Co-doped zinc oxysulfide Zn(O,S) solid solution with different amounts of Co (0–10%) was demonstrated for photocatalytic application. It aims to support green energy production and environmental remediation, which have gained increasing attention over the years. Specifically, the as-prepared nanocatalyst was utilized for photocatalytic hydrogen production and hydrogenation reaction. The oxysulfide formation of Zn(O,S) was proved by selected area electron diffraction (SAED), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS) analyses. Briefly, a Co content of 2.5% was found to be the most optimum value for the doping at Zn cation sites in the Zn(O,S) structure, as supported by the results of electrochemical impedance spectroscopy (EIS) and transient photocurrent (TPC) analyses as well as by hydrogen evolution reaction (HER) experiments. In good agreement, 2.5% Co-doped Zn(O,S) exhibited the highest amount of photoevolved hydrogen (H2) under either low-power UV light or solar light illumination, which were 27 000 and 1565 μmol g–1, respectively. Furthermore, before forming H2, the in situ generated proton (H+) by 2.5% Co-doped Zn(O,S) could be employed for the photocatalytic hydrogenation reaction (PHR) either to cleave the azo bond (N═N) in azobenzene or to reduce the nitro group (−NO2) in nitrobenzene. The UV light-induced conversions of AB and NB to aniline were completed within 30 and 120 min, respectively. Finally, the appropriate mechanisms for the HER and PHR, which involve molecular solvation, surface adsorption, molecular pinning, and hydrogenation reactions, are scrupulously proposed and discussed.